You need to increase the jet size, pull out the primary jet to get the size number. You'll need to drill it out or best is find some larger jets to try. I got my H/Ws from Langdons and Tom sent me some extra jets to try and that cured the problem.

I found a book on these carbs plus info searching the net.

Jim, I.I. #173
(It's easier to get forgiveness than permission!)